set password doesn't work

Hi,

I installed on linux the dbms "PostgreSQL 8.2.3 on i686-redhat-linux-gnu,
compiled by GCC gcc (GCC) 4.0.2 20051125 (Red Hat 4.0.2-8)".

Initially I set an empty password for the user postgres.

Some times later I connect from a different machine (Windows) to that
database, and I set a password for postgres user, and now I see it in
pgAdmin as:

CREATE ROLE postgres LOGIN
  ENCRYPTED PASSWORD 'md505a74e1678967a543fd92e711aa5762d'
  SUPERUSER INHERIT CREATEDB CREATEROLE;

Surprising I was able to connect now to that database server, without
password, or specifying any text as password, both from the different
machine, and from the same database server machine.

So then I tried to change the password by the following script:

ALTER USER postgres WITH ENCRYPTED PASSWORD '...';

But nothing change, and it has the same behaviour like an empty password.

What's wrong ? Please let me know if I applied the right procedure to set
the password for postgres user.

TIA,
Sabin